1. Field of Invention
This invention relates to variable torque/speed transmission, specifically to a variable transmission where the transmission ratio can be varied continuously between any two predetermined values.
2. Description of Prior Art
In most applications the transmission ratio, which is the torque vs. speed ratio transmitted by a driving source, needs to be adjustable in order for the driving source to operate efficiently and effectively. For example, for a vehicle, during start-up, assuming that it is on a level road, the driving source needs to provide torque to accelerate the vehicle and torque to overcome the resisting forces mainly due to friction and wind resistance. Once the vehicle has reached its desired speed, again assuming that it is on level road, the engine only needs to provide torque to overcome the resisting forces, which in this case is likely to be greater than during start-up, but less than the total torque needed during start-up. Hence in this case the torque that the driving source needs to provide is less than the torque that it needs to provide during start-up. However, here the driving source needs to rotate the output shaft at a higher speed since the desired speed of the vehicle is assumed to be greater than the speed of the vehicle during start-up. From the example above, it can be seen that during start-up, the driving source needs to provide a relatively large torque and operate at a relatively low speed. And once the desired speed is reached, the driving source needs to provide a relatively small torque and operate at a relatively high speed. Here a relatively large torque would be wasteful. Hence in order to increase the efficiency of the driving source most vehicles have a transmission, which can vary the torque vs. speed ratio of the driving source.
Most vehicles, such as cars, bikes, or motorcycles use a discrete variable transmission. Here the operator can select between several discrete transmission ratios usually by selecting an input gear or sprocket that is coupled to an output gear or sprocket, which is selected from a set of output gears or sprockets of various pitch diameters. The main advantage of a Continuous Variable Transmission (CVT) over a discrete variable transmission is that a CVT can provide the driving source with a more efficient transmission ratio under most conditions.
One well know CVT, which principal of operation is similar with many CVT's of prior art, consists of two cones, each keyed to a separate shaft, that are coupled by a belt. Because the cones have a tapered surface, the pitch diameters of the cones, which depend on the diameters of the surface of the cones where the belt is axially positioned, changes as the axial position of the belt is changed. Since the apex of the cones point in the opposite direction, changing the axial position of the belt increases the pitch diameter of one cone while decreases the pitch diameter of the other cone. This fact is used to change the transmission ratio between the shafts. One problem with this CVT is that changing the transmission ratio causes wear and frictional energy loses, since the belt has to slide and/or stretch relative to the surfaces of the cones as the pitch diameters are changed.
Another problem with the CVT mentioned in the previous paragraph is that torque can only be transmitted by friction. The need of friction limits the torque that can be transmitted, without causing unpractical high stresses in the belt and in the CVT's supporting members.